Known techniques for determining in real time the location of a cellular phone are based upon techniques such as direction finding (DF), time of arrival (TOA), and time difference of arrival (TDOA). These techniques, however, cannot accurately and reliably locate transmitters in severe multipath environments. An alternate approach to the location determination problem is disclosed by Hilsenrath et al. in U.S. patent application Ser. No. 08/780,565 now U.S. Pat. No. 6,026,304 (which is not admitted to be prior art with respect to the present invention by its mention in this Background section). This alternate approach, which takes advantage of multipath signals rather than attempting to mitigate or avoid them, measures a signal signature and determines a location by matching the signature with a calibrated database of signature-location pairs. In other words, rather than calculating a location from the measured signal signature, the location is determined by matching the signature with a calibrated signature in a database of known signature-location pairs.
An important step in practicing the above location-finding method involves generating, storing, and searching the database of calibrated signatures and associated locations. Jayaraman et al. in U.S. patent application Ser. No. 09/064,649 (which is not admitted to be prior art with respect to the present invention by its mention in this Background section) discloses a method for generating such a calibration table. In order to conserve data storage resources and improve the speed and efficiency of the location finding process, it is desirable to generate a compact and optimal signature-location database that may be quickly searched.